20.嵌套类型

　　枚举常被用于为特定类或结构体实现某些功能。类似的，枚举可以方便的定义工具类或结构体，从而为某个复杂的类型所使用。为了实现这种功能，Swift 允许你定义嵌套类型，可以在支持的类型中定义嵌套的枚举、类和结构体。

 　　要在一个类型中嵌套另一个类型，将嵌套类型的定义写在其外部类型的{}内，而且可以根据需要定义多级嵌套。

1.嵌套类型实践

struct BlackjackCard
{
    //嵌套的Suit
    enum Suit : Character
    {
        case Spades = "♠";
        case Hearts = "♡";
        case Diamonds = "♢";
        case Clubs = "♣";
    }
    
    //嵌套的Rank
    enum Rank : Int
    {
        case Two = 2, Three, Four, Five, Six, Seven, Eight, Nine, Ten;
        case Jack, Queen, King, Ace;
        
        //嵌套结构体Values
        struct Values
        {
            let first: Int;
            let second: Int?;
        }
        
        //属性
        var values: Values
        {
            switch self
            {
            case .Ace:
                return Values(first: 1, second: 11);
            case .Jack, .Queen, .King:
                return Values(first: 10, second: nil);
            default:
                return Values(first: self.rawValue, second: nil);
            }
        }
    }
    
    let suit: Suit;
    let rank: Rank;
    var description: String
    {
        var output = "suit is \(suit.rawValue),";
        output += " value is \(rank.values.first)";
        if let second = rank.values.second
        {
            output += " or \(second)";
        }
        return output;
    }
}

let theAceOfSpades = BlackjackCard(suit: .Spades, rank: .Ace);
print("theAceOfSpades: \(theAceOfSpades.description)");  //"theAceOfSpades: suit is ♠, value is 1 or 11\n"

//在外部引用嵌套类型时，在嵌套类型的类型名前加上其外部类型的类型名作为前缀
let heartsSymbol = BlackjackCard.Suit.Hearts.rawValue;